Electrocardiography (ECG) is a widespread commonly used and well known technique for the recording of the electrical activity of the heart over time. ECG measurements can therefore reliably support the diagnosis of heart failures, like congestive heart failure, abnormal heart rhythms for example caused by dyssynchrony of cardiac contraction, arterial fibrillation or arterial flutter. An ECG device records the electric impulses of the heart, which originate in the sinoarterial node and travel through the intrinsic conducting system to the heart muscle, are recorded over time. In conventional ECGs the electrical depolarization wavefront is usually measured via electrodes which are placed at selective locations on the skin of the patient. The ECG device then displays the voltage between pairs of these electrodes flooded over time. The standard ECG therefore describes the time dependent characteristics of the electrical activity of the heart. Depending on the application the ECG measurement data can also be used in a so called vector ECG to describe the spatial characteristics of the electrical heart activities. In other words, within a vector ECG the ECG measurement data are used for imaging the spatial propagation of the depolarization wavefront over time. The depolarization wavefront is thereby often imagined as a three-dimensional vector (usually denominated as mean electrical vector) which has at every point in time a defined direction (the direction of propagation) and a defined length (depending on the voltage drop at the wavefront).
For many applications where more precise diagnoses are needed standard ECG devices are not accurate enough. In these situations intracardiac ECGs are performed. An intracardiac ECG (also denoted as ECG mapping) measures the electric potentials within specific cardiac regions by placing electrodes within the heart via a cardiac catheter. This technique is especially applied when the electrical activity of the heart needs to be evaluated in regions within the cardiac conducting system, such as for example in the region around the bundle of HiS, where no ECG signals can be acquired using a standard ECG device with body surface electrodes. The accuracy of an intracardiac mapping is therefore far beyond standard ECGs. ECG mapping is thus a very important technique for the planning of a catheter ablation procedure, which is used to remove a faulty electrical pathway from the heart.
The main disadvantage of an intracardiac ECG is its necessary invasive procedure where a catheter is introduced into the patient's blood vessels which are advanced towards the heart, usually either through the femoral vein, the internal jugular vein or through the subclavian vein. This represents a serious surgical intervention which is not only complex and time consuming but also uncomfortable and not without risk for the patient.
A non-invasive measurement technique having a comparably high accuracy as the intracardiac ECG is unfortunately not known so far.
Magnetic Particle Imaging (MPI) is an emerging medical imaging modality. The first versions of MPI were two-dimensional in that they produced two-dimensional images. Future versions will be three-dimensional (3D). A time-dependent, or 4D, image of a non-static object can be created by combining a temporal sequence of 3D images to a movie, provided the object does not significantly change during the data acquisition for a single 3D image.
MPI is a reconstructive imaging method, like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Accordingly, an MP image of an object's volume of interest is generated in two steps. The first step, referred to as data acquisition, is performed using an MPI scanner. The MPI scanner has means to generate a static magnetic gradient field, called “selection field”, which has a single field free point (FFP) at the isocenter of the scanner. In addition, the scanner has means to generate a time-dependent, spatially nearly homogeneous magnetic field. Actually, this field is obtained by superposing a rapidly changing field with a small amplitude, called “drive field”, and a slowly varying field with a large amplitude, called “focus field”. By adding the time-dependent drive and focus fields to the static selection field, the FFP may be moved along a predetermined FFP trajectory throughout a volume of scanning surrounding the isocenter. The scanner also has an arrangement of one or more, e.g. three, receive coils and can record any voltages induced in these coils. For the data acquisition, the object to be imaged is placed in the scanner such that the object's volume of interest is enclosed by the scanner's field of view, which is a subset of the volume of scanning
The object must contain magnetic nanoparticles; if the object is an animal or a patient, a contrast agent containing such particles is administered to the animal or patient prior to the scan. During the data acquisition, the MPI scanner steers the FFP along a deliberately chosen trajectory that traces out the volume of scanning, or at least the field of view. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization. The changing magnetization of the nanoparticles induces a time dependent voltage in each of the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. The parameters that control the details of the data acquisition make up the scan protocol.
In the second step of the image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. The image is a discrete 3D array of data that represents a sampled approximation to the position-dependent concentration of the magnetic nanoparticles in the field of view. The reconstruction is generally performed by a computer, which executes a suitable computer program. Computer and computer program realize a reconstruction algorithm. The reconstruction algorithm is based on a mathematical model of the data acquisition. As with all reconstructive imaging methods, this model is an integral operator that acts on the acquired data; the reconstruction algorithm tries to undo, to the extent possible, the action of the model.
Such an MPI apparatus and method have the advantage that they can be used to examine arbitrary examination objects—e. g. human bodies—in a non-destructive manner and without causing any damage and with a high spatial resolution, both close to the surface and remote from the surface of the examination object. Such an arrangement and method are generally known and are first described in DE 101 51 778 A1 and in Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in nature, vol. 435, pp. 1214-1217. The arrangement and method for magnetic particle imaging (MPI) described in that publication take advantage of the non-linear magnetization curve of small magnetic particles.